GEOLOOICAL SURVEY BULlETIN 70 Platinium Group Elements and their host rocks in Tasmania: A summary review ANTHONYV. BROWN Tasmania Department ofMines INTRODUCTION Between 1925 and 1939 -490 kg of 'osmiridium', including -430 kg from Adamsfield, were registered. The remaining 55 kg were registered between 1940 and 1980. There are four factors in favour of exploring for Platinum Group Elements (PGE's) in Tasmania: The 'osmiridium' alloys were mined from plaoer deposits spatially associated with olivine-rich ultramafic rocks. Over • PGE's occur in the State, and the areas of occurrence are the last few years, studies of the various areas of ultramafic known; rocks in Tasmania have shown that there exists a potential for 'hard rock' sources of lPGE' s, associated with these high • Access to land for exploration is dominantly free of magnesian, olivine-rich ultramafic rocks, as well as a restriction; potential for deposits of PPGE's, associated with zones of chromite schlieren in pyroxene-rich ultramafic rocks (Brown • Industrial demands are expanding as new products are 1988; Carthew, Peck, 1990; Peck and Keays, found in which PGE's are used; etal., 1989a, b; 1990). • With changing political and economic situations is South Africa and Russia, traditional supplies are no longer During the late 1940s, exploration for copper-nickel certain. mineralisation in the Cuni area (2 kID southwest of Serpentine Hill- no. 7, fig. 1) revealed platinum and At present, Platinum Group Elements are currently being palladium in a medium to fine-grained association of used in the petrochemical and automobile industries, in magnetite, pyrite, pyrrhotite, pentlandite and chalcopyrite electronics and communications, for medical and dental associated with gabbroic rocks. purposes, in the space industry, in fuel cells, as well as for jewellery and investment bars and coins. Relevant land use and access issues, based on the Department of Mines Mineral Exploration Code of Practice (Bacon, In occurrence and association PGE' s can be divided into two 1991) are discussed in an appendix to this paper. groups: IPGE, being iridium (Ir), osmium (Os) and ruthenium (Ru); and PPGE, being platinum (Pt), palladium (Pd) and rhodium (Rh). Gold can be associated with both of KNOWN OCCURRENCE OF PGE'S AND these groups. ASSOCIATED MINERALISATON IN TASMANIA Platinum Group Minerals (PGM's) mined in Tasmania in the past were commonly known as 'osmiridium', an alloy of Os Of the ftfteen separate areas of ultramafic rocks which occur and Ir with minor amounts of other platinoids, especially Ru. in Tasmania, PGE's have been obtained from six. Four areas However, when detailed analyses were obtained it was found have produced significant amounts of alloy, with the other that a compositional range existed, and the alloys ranged two areas producing lesser volumes. A seventh area from being dominantly iridosmine to rutheniridosmine with contained PGE's in alluvial deposits derived from now only minor true osmiridium (Ford, 1981; Cabri and Harris, eroded ultramafic rocks. 1975). Recent work has shown the Tasmanian ultramafic host rocks The four main areas are: contain not only Ir-Os-Ru alloys butaIso Ru-(Ir-Os) sulphide and platinum, as native Pt, as alloys with other PGE's, or in • the Heazlewood River Complex (no. 3, fig. 1) - also complexes as sulphide minerals (Brown et aL, 1988; Creasy, know as 'Nineteen Mile Creek', which includes Fenton's 1990; Peck, 1990; Peck and Keays, 1990; Peck et al., 1992). Prospect and the Caudry's worlcings; In the late 19th century PGM's were obtained as a by-product • the Mt Stewart Complex (no. 4, fig. 1) - also known as of gold mining from alluvial and eluvial deposits associated Laughin's Creek or Ramsay's Prospec~ with the ultramafic-mafic complexes. Until c.1900, when a market for osmium and iridium developed, the grains were • the Wilson River Complex (no. 5, fig. 1) - in the north, considered a nuisance and were discarded. the Hartnan River area and in the middle the Wilson River area, and Barnes and Riley Creeks; Between 1910 and the end of 1980 approximately %5 kg of • the Adamsfield area (no. 14, fig. 1). Early reports on 'osmiridium' was recorded as being been mined from 'osmiridium' in the Adamsfield area can be found in Nye western Tasmania. Alluvial mining between 1910 and 1925 (1929). on the Heazlewood-Mt Stewart-Wilson River complexes (no. 3-5, fig. 1) accounted for -420 kg of this total. In 1925 The two areas with smaller output were Boyes River (no. 13, the Adamsfield deposits (no. 14, fig. 1) were discovered. fig. 1) and Rocky Boat Harbour (no. IS, fig. 1) areas. These BROWN, A. V. 1992. Tasmania's development environment. Bull. geoL Surv. Tasm 70:48-55. 48 TASMANIA - ANISLANDOFPOlENTIAL a/a 0, 20 co 60, KM Scm , I - NI I Mathinna Beds IOrd .· Oev·I POST-OEVONtAN COVER 1 Andersons Creek AUSTRALIA 2 Forth 3 Heaz/ewood River 4 Mt Stewort 5 Wilson River 6 Huskinon River 1··················;:;:;:;:;:;:;:;:;:;:;: 1 Ordovician - Devonian 7 Serpentine Hill {ocombrian-Cambrian 8 Dundas 9 Melvon Hill Mt Read volcanics and correlates 10 Trio I Harbour Precambrian-comparatively unmetomorphosed 11 Cape Sorell 12 Spero Bay Precambrian - metamorphosed 13 Boyes River Ultramafic- mafic (omplexes 14 Adam,field 15 Rocky Boat Harbour Figure 1 Schematic geological map of Tasmania showing locations of uttramafic-mafic complexes. Rock distribution after 1:500 000 Geological Map of Tasmania (1976). 49 GEOLOOICAL SURVEY BULLETIN 70 3/8 <, two bodies are now in the World Heritage Area and are, at present, unavaiJable for exploration. 5.0 The seventh area, containing Keygan and Betts Creeks, is on w top of the Meredith Granite (Jack and Groves, 1964). The !::: alluvial deposits were derived from remnants of ultramafic a: • rocks left after granitic emplacement Prior 10 emplacement, a 2 .9 UG2. BUSHVELD z 8TH AFRle"", • '+ _ _, t- the ultramafic rocks would bave formed a continuous belt o , + from the Heazlewood River area in the north, through the Mt I , ' , Stewart area, to the Wilson River area in the south. () 1.0 , . ,,' ~ + ''-.,: w " STilLWA~ COMPLEX, As well as the areas with bislOricai production, two further C) 0.5 GREECE MONTANA areas; the Huskisson River Complex (no. 6, fig. I), and the a. Trial Harbour ultramafic body (no. 10, fig. 1); are composed ~ of the same primary ultramafic rock type as the bodies from g whicb 'osmiridium' was produced. A recently completed a: 0.2 exploration program over the Huskisson River Ultramafic ~ Complex obtained between 20 and 190 ppb PI for soil w samples from eluvial deposits, as well as grains with varying C) 0.1 ~AN __ 0 " proportions of Os and lr ± Rb, and grains of PI, as PI-Fe, a. TURKEY __ 00 - • PI-Rb-Fe-Ni and PI intergrown with Os and lr (Creasy, 1990). ' " .. 0 .05 ..\ • ' ,' NORTH • ... CALIFORNI An average pattern for wbole rock PGE"t:Ontents from the ~ ',' Tasmanian ultramafic rocks was calculated from 42 rock .' " 0.02 " .. samples analyses for Brown et aL (1988). Wben this average ",ct pattern was plotted on a cbondrit.e-normalised PGE diagram " . (fig. 2), and compared with other areas of ultramafic rocks ':,, ', ~ 0.01 with known PGE production, the pattern bas a similar slope 11,. to samples from opbiolite bodies, but, in comparison to these, • the Tasmanian 'sample' bas anomalously bigh PI values. 0.005 .' Outside ultramafic complexes, PI and Pd bave been reported with Cu-Ni sulphide ore in the Cuni copper-nickel field, wbicb isjust to the west of Serpentine Hill (no. 7, fig. 1). The 0.002 ore is a magnetite-pyrite-pyrrbotite-pentlandite solid solution. The first reference from this area was by Reid 0.001 (1921), wbo recorded 0.1-D.16 ounces per ton platinum in a Os Ir Ru Rh PI Pd Cu-Ni ore sample, whicb also contained trace gold and -1.4 ELEMENT ounces per IOn silver. A sample of ore from a second mine in the field assayed -3 glt platinoid metals, with PI and Pd Figure 2 being determined as being in approximately equal proportions (Stillwell, 1946). A third sample from a nearby Chondr~e-nonnalised platinum-group element data pit also contains platinum, partly as native platinum and (after Page et al., 1984) showing average data for 42 partly as sperrylite (Hughes, 1965). Data obtained from the samples (after Brown et aI., 1988). old mine workings and later drilling indicates that the ore of the Cuni copper-nickel field is contained within, or associated with, gabbroic dykes or sills (Taylor and Burger, ULTRAMAFIC HOST ROCKS FOR THE PGE'S 1952; Robinson, 1958). AND ASSOCIATED MINERAU SATION In the nearby Serpentine Hill Ultramafic Complex, small, The arcuate belt of Eocambrian-Cambrian rocks in western cobalt-bearing pentlandite-millerite grains pervade the Tasmania comprises a number of different serpentinitic matrix of the dominantly orthopyroxene-ricb volcano-sedimentary successions, with the volcanic rocks ultramafic rocks. Olivine-ricb zones within this sequence, ranging from mafic to felsic. Along the deformed western consisting of plagioclase-bearing dunite and peridotite, margin of this zone a number of ultramafic-mafic rock contain cbromite grains with laurite [Ru(Os,lr)S2] and complexes are exposed (fig. 1). Os-lr-Ru alloy grains. Values for PI (-10-70 ppb) and Pd (-1-10 ppb) from wbole-rock samples from the different Using a combination of field characteristics and constituent ultramafic and gabbroic rocks were obtained from this area mineral cbemistty, the ultramafic rocks in these complexes (Brown et al. , 1988; Brown, 1991). The higbest values ofPd bave been subdivided into three different successions were obtained from the late-stage gabbroic samples. Values (Brown, 1986): for PI (W.5 ppb) and Pd (21.5 ppb)" were also obtained from wbole-rock samples of the associated low-Ti tholeiitic basalt.